无机纳米材料化学4.pdf

1 第四节 纳米材料的化学性能第四节 纳米材料的化学性能 纳米材料的光催化纳米材料的光催化 4 1 光催化原理简介光催化原理简介 光催化的内涵 是指在有光参与的条件下 发生在光催化剂 及其表面吸附物之间的一种光化学反应和氧 化 还原过程 在环境保护应用方面 近20多年来 用于 降解有机污染物的光催化剂多为N 型半导 体材料 如TiO2 ZnO CdS SnO2 Fe2O3等 纳米TiO2 因其具有活性高 稳定性好 对人体无害 成本低 并且可在常温常压 下工作等特性而作为重要的光催化剂 光催化的基本原理是 当半导体氧化物受到大于禁带宽度能量的光子照射后 电 子从价带跃迁到导带 产生了电子 空穴对 电子具有还 原性 空穴具有氧化性 空穴与氧化物半导体表面的OH 反 应生成氧化性很高的 OH自由基 活泼的 OH自由基可以把 许多难降解的有机物氧化为CO2和H2O等无机物 如 绝缘体金刚石禁带的能隙为5 2eV 半导体的禁带的能 隙为一般为0 3 eV左右 TiO2的能带位置与被吸附物质的还原电势 决定了其光催化反应的能 力 热力学允许的光催化氧化 还原反应 要求受体电势比TiO2导带 电势更正 给体电势比TiO2价带电势更负 才能发生氧化 还原反 应 例如 如果光生电子能够还原水 H H2 那么导带的电势必须低 于E H H2 同样如果光生空穴能够氧化水 那么 价带的电势必定 要大于E O2 H2O 2 实验表明 TiO2表面的空穴具有大的反应活 性 它可以将吸附在表面的OH 和H2O分子氧化形 成具有很强氧化性的羟基自由基 OH OH能氧 化绝大多数有机污染物和部分无机污染物 将其 最终降解为CO2 H2O等无害物质 此外 许多污 染物也可能直接被空穴所氧化 OH h OH H2O h OH H 3 4 2 纳米半导体材料的光催化特性纳米半导体材料的光催化特性 4 a b a b 100 nm100 nm c Fig 2 a Representative SEM images b high magnification SEM images c TEM images inserted a selected area electron diffraction pattern of the Mn3O4 nanoplates prepared by oxidation reflux method at 80oC for 3h Fig 4 a FE SEM panoramic images b FE SEM magnification images c TEM images inserted ED pattern of Mn3O4 nanoparticles as prepared through solvothermal process at 180 oC for 14h KAM Ahmed KX Huang Chemical Engineering Journal 172 2011 531 539 Fig 5 Cyclic voltammograms of Mn3O4nanocrystals modified CPE electrodes a comparison of nanoplate modified electrode NL with CPE electrode b comparison of nanoparticle modified electrode NP with CPE electrode c comparison of NL with NP 0 0 02 0 04 0 06 0 08 0 1 020406080100 Time min At A0 a III II I 0 0 05 0 1 0 15 0 2 0 25 0 3 0 35 020406080100 Time min lnAt A0 b Fig 6 The oxidation of formaldehyde by Mn3O4nanocrystals in aqueous solutions a CH2O O2 I CH2O O2 Mn3O4nanoparticles II CH2O O2 Mn3O4 nanoplates III b the oxidation rate of formaldehyde oxidation to formic acid by Mn3O4nanoplates KAM Ahmed KX Huang Chemical Engineering Journal 172 2011 531 539 a bc abc d HA Abbood KX Huang et al Chemical Engineering Journal 209 2012 245 254 Fig 2 SEM images of the cross like NH4V4O10nanobelt arrays fabricated by a hydrothermal process at 160 C for 6h a Low magnification SEM image of large number crossing structure b high magnification SEM image of two crossing like nanostructures c a close up view of connection center of as prepared nanobelt arrays d a photograph of a bamboo mat Fig 3 TEM images of the cross like NH4V4O10nanobelt arrays fabricated by a hydrothermal process at 160 C for 6h a TEM image of a typical cross parts Inset a FFT pattern b TEM image of the end of cross like nanobelt arrays c SAED patterns d HR TEM image of as prepared cross like nanobelt arrays at cross center b 0306090120150180 210 0 0 0 3 0 6 0 9 1 2 1 5 1 8 2 1 Abs a u Time min Without catalyst Befor cacination P25 Aftert calcination 0 20 40 60 80 100 350C550C75 0C D Temperature 0C c 0306090120150180210 0 0 0 5 1 0 1 5 2 0 2 5 3 0 ln C0 C Time min After cacination P25 d 200250300350400450500550600650700 0 00 0 25 0 50 0 75 1 00 1 25 1 50 1 75 2 00 2 25 Abs a u Wavelength nm 0 min 60min 120min 180min 210min a Fig 13 a UV vis spectra of degradation products of rhodamine B in aqueous solution with O2 and under light irradiation catalyzed by mixed valance state vanadium oxide nanobelt arrays in different times b Comparison of the rhodamine B degradation with times catalyzed by mixed valance state vanadium oxide nanobelt arrays cross like NH4V4O10 nanobelt arrays Degussa P25 TiO2and without catalyst c Kinetics of the rhodamine B degradation catalyzed by mixed valance state vanadium oxide nanobelt arrays and Degussa P25 TiO2 at 75 C d